A heliostat is a device generally provided in a fixed location that includes a reflective surface to reflect solar energy toward a predetermined, fixed target. A heliostat, or more accurately an array of heliostats, is most commonly used in concentrating solar power installations.
A heliostat includes a tracking mechanism which tracks the movement of the sun to maintain the reflected image on the target. To achieve this, a line normal to the centre of the reflective surface must generally bisect the angle formed between the sun, the reflective surface, and the target. As a result, the angle of incidence of sunlight upon the reflective surface varies with the time of day and year.
The reflective surface of a heliostat may be planar, but is more commonly concave to concentrate reflected solar rays towards a focal point which is generally at or adjacent the target. U.S. Pat. No. 3,892,433 to Blake, for example, discloses a system including an array of spherical mirrors reflecting light towards a heat receiver which generates superheated steam to drive a steam turbine generator.
In a focusing heliostat, astigmatism causes the size of the reflected image to vary throughout the day, becoming larger as the angle of incidence (with respect to the normal) increases. A larger image reduces the energy density and, if the image is larger than an aperture in the target, results in wasted energy and reduced efficiency. Conversely, if the image is smaller than the aperture, the excess area of the aperture increases heat loss to the atmosphere.
It has been demonstrated by Igel & Hughes (Igel, E. A., & Hughes, R. L. (1979). Optical analysis of solar facility heliostats. Solar Energy, 22, 283-295) that the effects of astigmatism can be minimised for extended periods of time during the day by using non-imaging optics (i.e. an asymmetric reflector).
U.S. Pat. No. 4,459,972 to Moore discloses a concave focusing reflector which is formed as a section of a toroid defined by two unequal radii of curvature in mutually perpendicular planes. The toroidal heliostat reflector provides a well-defined concentrated image with power distribution and size which varies less than that of a spherical mirror throughout the operational day and year. If the plane formed by the angle between the incident and reflected rays is maintained such that its intersection with the mirror remains substantially fixed in use, aberrations in the image are reduced.
Toroidal heliostats differ from spherical heliostats in that they have different focal lengths for each of the different axes (generally referred to as the tangential and sagittal directions or axes). The ability to optimise these focal lengths independently (i.e. by adjusting the radii of curvature) is the key to what makes the toroidal heliostat more effective than a spherical heliostat.
The toroidal heliostat shape is an approximation of the ideal paraboloidal shape that would be constructed were the target considered as the focal point of a large paraboloidal surface intersecting with the heliostat position. The toroidal shape approximates the curvature of the paraboloidal surface with the tangential axis aligned with the focal axis of the paraboloidal surface, and the sagittal direction perpendicular to it.
Chen et al. (Chen, Y. T., Chong, K. K., Bligh, T. P., Chen, L. C., Yunus, J., Kannan, K. S., . . . Tan, K. K. (2001). Non-imaging, focusing heliostat. Solar Energy, 71(3), 155-164) discloses a variation of the toroidal heliostat in which the reflective surface comprises a plurality of planar mirrors or facets forming a pseudo-toroidal surface. Each row and column of mirrors can be individually pivoted to achieve variable focusing to ameliorate the image aberration when combined with a spinning elevation or rotation-elevation tracking method (as opposed to the more common altazimuth or equatorial heliostat mounts of the prior art). However, the heliostat of Chen et al. requires means for rotating not only the heliostat frame about two axes, but also means for adjusting the angle of each row and column of mirrors mounted to the frame, contributing significantly to the complexity and cost of the heliostat.
Heliostats are generally the most expensive component of a concentrating solar power installation. It is therefore desirable to provide a heliostat which leads to improved efficiency and/or reduced manufacturing and maintenance costs to generate electricity at lower cost.